The present invention relates to a method of bonding polyester to plastic and more particularly, to a method of bonding a fiberglass reinforced polyester resin to polyethylene.
Polyethylene is well known to be a synthetic thermoplastic material having excellent properties of chemical inertness and water resistance. However, polyethylene has a very low modulus of elasticity (Young""s modulus), or low rigidity, and ability to act as a support structure.
The chemical inertness of polyethylene, attributable to its highly non-polar properties, makes layers of this material well suited for lining containers, including bulk holding tanks, and other articles to be employed in contact with substances that chemically attack plastics or metals.
However, the low modulus of elasticity of polyethylene severely limits the rigidity, and consequently the size, economy, or serviceability of liquid storage tanks and other articles made of this material. For example, as the size of tanks is increased, it is necessary to either make the tanks with very thick walls of polyethylene, or to support walls of the tanks in rigid frames which themselves are not sufficiently resistant to chemical corrosion. In other applications, polyethylene components mounted to the internal walls of the tanks, typically cannot be fully self-supported unless they are very thick. In all of these applications, the cost involved causes polyethylene structures to be commercially undesirable.
Separately, it is well known that polyester resins possess a relatively high modulus of elasticity, or rigidity but have a lower resistance to chemicals than the resistance offered by polyethylene. Typical examples of polyester resins are copolymers of dicarboxylic acid esters of polyvalent alcohols, vinyl esters and vinyl monomers. They are often compounded with reinforced materials, especially glass fiber, typically referred to as fiberglass; however, even with these enforcing materials, the resins have a lower resistance to chemicals than polyethylene. Other materials may be incorporated into the polyester resins as will be appreciated by the art, including but not limited to, fibers such as cotton or shredded fabrics, mineral fibers, such as asbestos, and finely divided fillers or pigments.
Unfortunately, polyester resins, including reinforced resins such as fiberglass, have a relatively high polarity, and therefore coatings or layers of this material are not directly adhereable to layers of polyethylene by conventional coating or molding techniques without resorting to costly treatment of the polyethylene layer, such, for example, as a flamed treatment to oxidize its outer surface. Another conventional method of combining the two materials in a single structure as required the deposition of a layer of acrylic material, such as an acrylic resin on the surface of the plastic layer. A layer of fiberglass is then deposited on the acrylic layer on opposite side from the plastic layer. A large drawback to using these methods is that they involve complicated and costly manufacturing procedures to effectively bond polyester resin to polyethylene.
The aforementioned problems are overcome in the present invention using a method that provides a strong interfacial mechanical bond between plastic and polyester layers, notwithstanding the widely different chemical and physical properties of the two materials.
In the method of the present invention, a surface of a layer or body of plastic, for example, polyethylene, is physically ground such that a plurality of tiny plastic hairs protrude from the surface of the plastic layer or body. Next, a coating or layer of a polyester resin, such as fiberglass is applied or deposited on the ground surface of the plastic. Subsequently, the polyester resin hardens and effectively encapsulates and surrounds the plastic hairs to form a strong interfacial mechanical bond between the two layers.
As a consequence, rigidified polyethylene structures can be readily produced in the form of containers, bulk tanks, components for tanks, panels, and various other desired articles, with the body of each article including a layer of reinforced polyester resin directly mechanically bonded to the polyethylene layer. If desired, the properties of the polyester resin may be realized on both sides of a polyethylene article by simply grinding both sides of the article and depositing polyester resin layers thereon. Alternatively, the properties of the polyethylene may be realized on multiple sides of a polyester resin by facing two ground surfaces of polyethylene toward each other and depositing the polyester resin in a sandwich-like fashion therebetween. The articles including such a polyethylene/polyester resin wall structure can be produced economically in various configurations and sizes.
These and other objects, advantages and features of the invention will be more readily understood and appreciated with reference to the detailed description of the preferred embodiments and drawings.